Hook : Java Glossary

In the old days, the telephone mouthpiece rested on hook-shaped cradle when it was
not in use. To use the phone, you lifted the mouthpiece off-hook which gave you a dial tone. To stop using the phone, you
placed it back on-hook . We still use these terms for the
electrical modem equivalents.

A hook is also a place in standard computer code where you can insert a piece of
custom code. In Java, this is usually implemented with a delegate object.